LEDs are used as backlight sources by more and more LCD devices, with the maturing of LED technology and the cost reduction of the LEDs. An LED driver is different from a conventional power supply driver, and generally adopts the pulse-width modulation (PWM) technology to regulate the brightness. As shown in FIG. 1, the LED driver is in a dimming state, when the PWM of a dimming signal is ON, a switch tube of a power supply end is conducted and the energy is transported to a back-end capacitor CP2 and an LED lightbar; when the PWM of the dimming signal is OFF, the driving circuit is out of operation, and at the moment, the LEDs also go out.
When the PWM is OFF for a too long time, the charges of the capacitor will still be consumed by the leakage current of the LEDs, and at the moment, more energy is required for charging the LEDs and the capacitor when the PWM is ON. Thus, when the PWM is ON for a too short time, the required energy cannot be achieved, namely, the LEDs will not operate within a shorter dimming cycle.